Wireless communications devices have been developed for a wide variety of applications. A hearing instrument, such as a wireless digital hearing aid (WDHA) or listening device to assist with hearing, is one of the devices.
The WDHA may include circuitry for audio input and output, a digital signal processor (DSP), one or more signal co-processors, non-volatile storage, analog radio frequency (RF) circuitry for RF transmission and reception, and circuitry for baseband digital coding and modulation and demodulation. These components may be implemented on the WDHA in one or more integrated circuits (ICs).
The signal flow of a WDHA transmitter is as follows: An audio signal is received through an electrical transceiver and connected to the analog circuits to convert the analog audio signal into a digital signal. The digital signal is stored in temporary RAM on the WDHA. Through a combination of software-based and hardware-based processing the digital signal is processed and passed to the analog RF circuitry for RF transmission.
The signal flow of a WDHA receiver is as follows: The analog RF circuitry receives an RF transmission and converts the signal to a digital baseband signal. The digital signal is processed through a combination of software-based and hardware-based processing. The digital signal may be combined with other digital signals sourced from electrical transceivers on the receiver. The digital signal is then passed to the analog circuits to convert to an analog signal. An electrical transducer converts the analog signal into sound energy for the WDHA user.
In the processing stages of the transmitter and receiver the WDHA may manipulate the signals in a number of ways to compensate for a WDHA user's hearing loss profile. This may include processing on the local signals as well as on transmitted or received signals.
A WDHA system may consist of two WDHAs. One WDHA is designated as a transmitter. The other WDHA is designated as the receiver. The transmitter may transmit audio data or other data to the receiver. The transmitted data, including audio data, may have been previously processed or compressed according to the bandwidth limitations of the wireless radio link (RF link). On the receiver the received data may be further processed and subsequently mixed with local audio data to generate the final outgoing audio signal for the WDHA user. Non-audio data may be used for control purposes.
Another WDHA system may include a non-hearing aid based transmitter and a receiving WDHA. The transmitter may be stationary (fixed) or portable. The transmitter in this case may be connected to some other stationary or portable audio device such as a music player, computer, or television. The transmitted data, including audio data, may be pre-processed or compressed according to the bandwidth limitations of the wireless radio link. On the receiver the received data may be further processed, including decompression, and subsequently mixed with local audio data to generate the final outgoing audio signal for the hearing aid user. Non-audio data may be used for control purposes, such as volume control or program selection.
In order to provide sufficient or better audio quality to the WDHA user, there must be enough bandwidth available on the wireless radio link to send the audio signal with minimal distortion and minimal noise. Also, since the WDHA must operate on a battery with limited voltage and current capabilities, the wireless radio link must operate using minimal power to maximize battery life. Due to these two opposite requirements, a tradeoff must be made between bandwidth and power consumption. Thus the audio signal quality is a function of the available bandwidth and increased bandwidth leads to decreased battery performance.
In addition to battery performance requirements, available bandwidth may be limited by regulatory limitations and physical antenna size limitations.
In order to maximize the audio quality of the WDHA the bandwidth allocated to audio data over the wireless radio link must be maximized.
Digital data to be transmitted includes sequences of digital bits (1's and 0's). Together the digital bits form blocks of digital data that are interpreted as values on the receiver of the wireless digital hearing aid. In order for the received data to be meaningful it requires a bit reference (referred to herein as a bit clock) and frame reference (referred to herein as a frame signal). The bit clock edges are ideally synchronized with the bit boundaries and can thus be used to extract the value of each individual bit. The frame signal is ideally synchronized with the frame boundaries and can thus be used to extract the bits associated with each frame. By using the bit reference and the frame signal, the receiving hearing aid is able to interpret the incoming data stream as frames of data. Data streams without a corresponding bit clock or frame signal cannot be properly interpreted.
In ideal wired communication systems the bit clock is transmitted in parallel with the data. This method provides the best synchronization between the data and clock but requires twice the bandwidth of the data alone because both need to be transmitted and received simultaneously. In order to reduce the bandwidth requirements for digital storage systems or wired communication systems coding techniques are used to embed the bit clock into the data signal. One such coding technique is known as Miller coding (U.S. Pat. No. 3,108,261 to Miller). Similar coding techniques may be used in wireless systems.
Further, in ideal wired communication systems the frame signal is transmitted in parallel with the data. This method provides the best synchronization between the data and the frame signal but requires twice the bandwidth of the data alone.
A typical alternative to transmitting the frame signal in parallel with the data is to create the frame signal on the receiver based on the incoming data. A technique for doing this is as follows: The transmitter inserts a known sequence of bits into the data stream to signify the start (or end) of a frame. The sequence occurs either at a predetermined location in the data stream or at some other place in the data stream that both the transmitter and receiver agree upon. When the sequence occurs at the start of the frame it is commonly referred to as a preamble. For the purposes of this discussion the known sequence occurring anywhere in the sequence is referred to as the preamble.
When using the preamble, the receiver must continually scan the incoming data stream for the preamble. Once the preamble is received the receiver can establish a reference.
The preamble technique is effective but has many limitations. The transmitter must ensure that the preamble does not occur in the regular data stream. Increasing the length of the preamble minimizes the probability of such an occurrence. A longer preamble will reduce the bandwidth available for the audio signal. Further, the transmitter may monitor the regular data stream and if the stream contains the preamble, it may modify it or add an escape sequence to prevent the receiver from detecting a preamble when it shouldn't.
In a WDHA the transmitter and receiver operate using different clocks. Therefore, since the bit clock frequency may drift over time the preamble must be sent periodically to properly synchronize the frame signal on the receiver. Since the preamble is inserted in the data stream, the space allocated to the preamble may not be used for other purposes such as audio data. Further, in a synchronous audio system with a fixed audio data rate the audio data stream may not be interrupted. If the data stream is interrupted, the audio data becomes non-continuous and results in a noticeable audio artifact that is heard by the hearing aid user. Such an artifact is undesirable. Typically, to avoid interruptions due to the necessary preamble, each data frame will contain the preamble or otherwise allocate space for the preamble that may not be used for regular audio data. In other words, the bandwidth required between the two systems will be higher than the net throughput once the preamble has been removed. Thus, the preamble uses valuable bandwidth that may otherwise be used for payload data such as audio. This causes the audio quality to be degraded compared to a signal with no preamble.